Plate Tectonics
EQ: What is the theory of plate
tectonics?
EQ: What is the theory
of plate tectonics?
Plates
• A section of the lithosphere that
slowly moves, carrying pieces of
continental and oceanic crust
• The plates fit closely together along
cracks in the crust
• Plates carry the continents or parts
of the ocean floor, or both
Scientific Theory
• A well – tested concept that explains
a wide range of observations
EQ: What is the theory of
plate tectonics?
Plate Tectonics
• A geological theory that states that
pieces of Earth’s crust are in
constant, slow motion, driven by
convection currents in the mantle
• Convection Currents: The movement of a
fluid, caused by differences in temperature
and density, that transfers heat from one
part of the fluid to another
EQ: What is the theory of plate
tectonics?
How can Earth’s plates move?
• The plates of the lithosphere float on
the top of the asthenosphere
• Convection currents rise in the
asthenosphere and spread out
beneath the lithosphere
• Geologists believe this causes plate
movement
EQ: What is the theory of
plate tectonics?
As plates move, they collide, pull
apart, or grind past each other,
producing spectacular changes in
the earth’s surface.
EQ: What is the theory of plate tectonics?
These changes include
volcanoes, mountain ranges,
deep sea trenches, and
earthquakes.
EQ: What is the theory of
plate tectonics?
Figure 26: Interpreting Maps
Which plates include only ocean
floor? Which plates include both
continents and ocean floor?
• Plates that include
only ocean floor are
the Pacific Plate
and the Nazca
Plate
• All other labeled
plates in the figure
include both
continents and
ocean floor
EQ: What is the theory of plate tectonics?
Study the map of Earth’s plates in
Figure 26. Notice the arrows that
show the direction of plate movement.
Now find the Nazca plate on the map.
What direction is it moving?
• The Nazca plate
is moving east
EQ: What is
the theory of
plate tectonics?
Find the South American
plate and describe its
movement.
• The South
American plate
is moving west
EQ: What is the
theory of plate
tectonics?
What do you think will happen
as these plates continue to
move?
• If the plates continue
to move, they will
collide (convergent
boundary)
EQ: What is the
theory of plate
• Since the Nazca plate
carries oceanic crust
and the South
American plate carries
continental crust, the
Nazca will plunge
beneath the South
American plate
Faults
• A break in the Earth’s crust where
slabs of rock slip past each other
EQ: What is the theory of
plate tectonics?
three types of plate
boundaries
EQ: What
is the
theory of
plate
tectonics?
• Transform Boundaries
• Divergent Boundaries
• Convergent Boundaries
• At each type of plate boundary, there is
a different type of plate movement
Transform Boundaries
• A plate boundary
where two plates
move past each
other in opposite
directions
EQ: What is the theory
of plate tectonics?
• Crust is neither
created nor
destroyed
What Earth changing event
occurs on a transforming
boundary?
• Earthquakes frequently occur at
transform boundaries
EQ: What is the theory of plate
tectonics?
Divergent boundaries
• A plate boundary where two
plates move away from each
other
• Most divergent boundaries
occur at the mid-ocean ridge
• Divergent boundaries that
occur on land are called rift
valleys
• Example: Great Rift Valley
(South Africa)
– A 3,000 km crack in the earth
EQ: What is the theory of plate
tectonics?
Rift Valleys
EQ: What is the
theory of plate
tectonics?
• A rift valley is a deep valley that
forms along a divergent boundary on
land
• Rift valleys form as two slabs of
Earth’s crust slide apart
What could eventually happen
at the Great Rift Valley in
Africa?
• The rift may someday split the
eastern part of Africa away from the
rest of the continent
EQ: What is the theory of
plate tectonics?
Convergent Boundaries
• A plate boundary where two plates
move toward each other
• Collisions can occur between:
– oceanic crust and oceanic crust
– oceanic crust and continental crust
– continental crust and continental crust
EQ: What is the theory of
plate tectonics?
EQ: What is the
theory of plate
tectonics?
When two plates collide, the
density of the plates determines
which one comes out on top.
What occurs when plates
carrying oceanic crust collide?
• The more dense plate dives back into
the mantle (subduction)
• The collision between oceanic plates
causes deep-ocean trenches
EQ: What is the
theory of plate
tectonics?
What occurs when plates
carrying oceanic and
continental crust collide?
• The more dense oceanic crust sinks
and plunges (subducts) beneath the
less dense continental crust
• The collision between oceanic plates
and continental plates can cause a
volcanic eruption
EQ: What is the theory
of plate tectonics?
What occurs when plates
carrying continental crust
collide?
• Plates crash head – on causing mighty
mountain ranges
EQ: What is the
theory of plate
tectonics?
What types of plate movement
occur at plate boundaries?
• Plates slip past each other at
transform boundaries
• Move apart at divergent boundaries
• Come together at convergent
boundaries
EQ: What is the
theory of plate
tectonics?
Plates move at a rate of
between one and ten
centimeters per year.
EQ: What is the theory of
plate tectonics?
EQ: What is
the theory of
plate tectonics?
The North American and Eurasian
plates float away from each other
at a rate of fingernail growth. This
is a rate of 2.5 centimeters per
year.
Figure 29: Posing Questions
What questions would you need
to answer in order to predict
where the continents will be in
50 million years?
• You would need
answers to how
fast and in what
direction each
plate is moving
EQ: What is the theory of
plate tectonics?